Find the file: c:\Program Files (x86)\Windows Kits\10\SDKManifest.xml
Inside the file, you will find the SDK version:
<FileList
DisplayName = "Universal Windows"
PlatformIdentity = "UAP, Version=10.0.22621.0"
TargetFramework = ".NETCore,version=v4.5.3;.NETFramework,version=v4.5.3"
MinVSVersion = "14.0"
MinOSVersion = "6.1"
MaxOSVersionTested = "10.0"
UnsupportedDowntarget = "Windows, version=8.1">
<File Reference = "Windows">
<ToolboxItems VSCategory = "Toolbox.Default"/>
</File>
</FileList>